1. Field of the Invention
This invention relates to a rare earth metal-series alloy for storage of hydrogen capable of forming a metal hydride at a temperature of -20.degree. C.-+80.degree. C. under a hydrogen pressure of 0.1-10 atm to efficiently absorb and release a large amount of hydrogen and having an improved resistance to poisoning through impurity gas, and more particularly to a rare earth metal-series hydrogen absorption alloy suitable for use in a heat pump, a hydrogen getter, a negative member of hydrogen storage battery, a hydrogen storage member and the like.
2. Related Art Statement
Hydrogen is an element rich in natural source and produces only water even through burning, so that the balance in ecology is not collapsed and the storage and transportation are easy. For this end, hydrogen is considered to mainly take a secondary energy in a promising clean energy system.
However, hydrogen is a gas at room temperature and is very low in the liquefying temperature, so that it is urgently demanded to develop a technique of effectively storing hydrogen.
As a technique responding to the above demand, there is noticed a system of storing hydrogen in form of a metal hydride. According to this system, hydrogen can be stored in the same weight as in commercially available hydrogen cylinder of 150 atmospheric pressure at a volume corresponding to not more than 0.2 times of the volume of the above cylinder, and also the safety and the handling are very excellent.
The material suitable for absorbing hydrogen in form of a metal hydride and releasing it, if necessary, as mentioned above is a hydrogen absorption alloy. It is strongly attempted to develop application systems over a wide range, i.e. it is attempted to conduct the development of heat regeneration system or heat pump by utilizing the generation or absorption of reaction heat accompanied with the formation or decomposition reaction of metal hydride in such absorption and release of hydrogen, or the development of metal oxide-hydrogen storage battery utilizing electrochemical reaction as a battery negative pole.
As the properties required for such a hydrogen absorption alloy, there are mentioned the followings:
(1) it is cheap and rich as resource; PA1 (2) it has a large hydrogen absorption capability; PA1 (3) it has a suitable hydrogen absorption and dissociation equilibrium at service temperature region and is less in the hysteresis indicating a difference between absorption pressure and dissociation pressure; PA1 (4) the hydrogen absorption and release reactions are reversible and the rate thereof is large; and the like.
There have hitherto been known MmNi.sub.5 (Mm is a Mischmetal), TiFe and the like as a typical hydrogen absorption alloy.
However, MmNi.sub.5 is required not only to take a high hydrogen pressure of 80-90 atmospheric pressure or a long treating time in the activation or initial hydrogenation but also to repeatedly conduct the above activation treatment, and has a drawback that the absorption and release of hydrogen take a long time. In order to solve such a problem, Mischmetal-nickel quaternary alloy (e.g. MmNi.sub.5-x Al.sub.x-y Fe.sub.y, wherein x is 0.1-2, and y is 0.01-1.99) has been proposed in Japanese Patent Application Publication No. 58-39217 and No. 59-28626.
On the other hand, TiFe has a drawback that when water, O.sub.2, CO, CO.sub.2 or the like incorporates into hydrogen, the alloy surface is poisoned with these impurities to largely lower the hydrogen absorption capability. As a solution on this problem, a technique of coating the surface of the hydrogen absorption alloy with a different metal through plating has been proposed in Japanese Patent laid open No. 58-1032. According to this technique, the surface of TiFe alloy hardly activated is coated with a metal such as Ni, Cu, Co or the like as an oxide relatively easily reduced in a hydrogen atmosphere through plating. Though it is required to repeatedly conduct the vacuum discharge treatment at a high temperature of 450.degree.-500.degree. C. and the pressurizing treatment (room temperature) through high pressure hydrogen gas under 30-60 atmospheric pressure for about 1 week as the conventional activation operation, the above technique reduces the treating temperature to not higher than 200.degree. C., the hydrogen pressure to 20-30 atmospheric pressure, and the treating time to not more than 1 day, respectively.
In the alloys described in Japanese Patent Application Publication No. 58-39217 and No. 59-28626 (e.g. MmNi.sub.3.7 Al.sub.0.5 Fe.sub.0.8), however, the equilibrium hydrogen absorption.dissociation pressure at room temperature is about 1 atmospheric pressure, the range progressing the hydrogen absorption, or a plateau on hydrogen pressure-hydrogen composition curve (temperature is constant) is flat and the hysteresis is small, but the hydrogen absorption quantity is undesirably small.
In the alloy obtained by the method of Japanese Patent laid open No. 58-1032, the activation is improved, but it can not be said that this alloy provides the sufficient properties as a hydrogen absorption alloy.
In all of the above conventional alloys, the resistance to poisoning through impurity gas is not said to be sufficient, and the decrease of hydrogen absorption capability with lapse of time is unavoidable.